Mixed signal integrated circuit (IC) chips that include both analog and digital circuit blocks often suffer from noise issues. Devices in the digital circuit blocks are typically noisy. In contrast, devices in the analog circuit blocks, such as phase-locked loops and low noise amplifiers, are usually sensitive to noise. The sensitivity of some analog circuit blocks, such as those in radio frequency ICs (RFICs), is heightened by the presence of on-chip passive elements, such as inductors. Efforts are typically made to protect such circuits from substrate, power/ground, and other electromagnetic interference.
Interest in minimizing the costs of mixed signal IC chips has resulted in increases in device density. The noisy devices and noise-sensitive devices are thus more likely to be positioned close to one another. Without proper noise isolation, noise generated by the noisy digital circuit blocks can interfere with sensitive analog circuit blocks.
Noise isolation is often provided by a conductive shield positioned between the analog and digital circuit blocks. One type of conductive shield is a grounded guard ring. In a typical arrangement, the grounded guard ring surrounds an analog circuit block to prevent the electromagnetic coupling of the noise to an inductor of the analog circuit block.
With the analog circuit blocks and digital circuit blocks formed in the same substrate, noise may also pass through the substrate. Substrate coupling allows noise at a node in a digital circuit block to reach a node in an analog circuit block. Noise isolation regions are accordingly formed in the substrate between the analog and digital circuit blocks to prevent such substrate coupling.